1. Field of the Invention
The invention relates to a method for the preparation of amorphous silibinin (or its synonyms: silybin or silibin) derived from a milk thistle fruit extract having an increased release rate and improved absorbability or bioavailability, and to the use of amorphous silibinin for the treatment or prevention of liver diseases, preferably for the treatment of viral hepatitis, e.g. hepatitis B or C, in particular in a patient who will undergo or has undergone liver transplantation. Preferably, the amorphous silibinin is adapted for oral administration.
2. Description of Related Art
The milk thistle (Silybum marianum or Carduus marianus) is a plant which is cultivated in particular in southwest and central Europe (Austria, Hungary), and which has become naturalized in Eurasia, North America, South America, and Australia. Production areas are also found in China.
Silymarin is contained in the dried ripe fruit of Silybum marianum (L.) Gaertneri (Fam. Asteraceaea) from which the pappus has been removed and which has a minimum silymarin content of 1.5% (Pharmacopoea Europaea (hereinafter: Ph. Eur.), 2007). Tinctures (usually alcoholic extracts) made from milk thistle have been known since ancient times. Isolated silymarin is particularly suitable (for example, DE 1 923 983, DE 1 767 666 (Madaus)). The efficacy of milk thistle (seeds or fruits) in the treatment and prevention of various forms of liver and gall bladder dysfunctions is known.
Silymarin is a complex of flavonolignans, i.e., polyhydroxyphenylchromanones, and was first isolated from the plant in the 1960s (Dissertation, Janiak Bernhard, June 1960, Berlin University of Applied Sciences (DE 2020407), Pelter A., Hansel R., Tetrahedron Letters, 25, 1968.
Silibinin {3,5,7-trihydroxy-2-(3-(3-hydroxy-4-methoxy-phenyl)-2-(hydroxymethyl)-2,3-dihydro-benzo[b][1,4]-dioxin-6-yl)chroman-4-one; or according to Ph. Eur. (2R,3R)-3,5,7-trihydroxy-2-[(2R,3R)-3-(4-hydroxy-3-methoxyphenyl)-2-(hydroxymethyl)-2,3-dihydro-1,4-benzo-dioxin-6-yl]-2,3-dihydro-4H-1-benzopyran-4-one} is the main constituent of silymarin and the main flavonolignan extracted from milk thistle (Silybum marianum Gaertneri).
Silibinin has the following structure:

The diastereomers silibinin A and silibinin B are distinguished in the literature:

Silibinin, the main constituent of silymarin, is usually present in an about 50:50 mixture of Silybin A and Silybin B. Further constituents of silymarin include isosilibinin (isosilybin A and isosilybin B), silidianin (silydianin), silicristin (silycristin), isosilycristin, taxifolin and others, such as known secondary components including dehydrosilibin, 3-desoxysilicristin, desoxysilidianin (silymonin), silyadrin, silybinom, silyermin, and neosilymerin. The primary constituents are the four flavonolignans silibinin, silidianin, and silicristin as well as isosilibinin. In these flavonolignan structures, taxifolin is linked to conifery alcohol. Methods for isolating silibinin are known from the prior art (e.g., U.S. Pat. No. 4,871,763; D. Y.-W. Lee et al., J. Nat. Prod. 2003, 66, 1171-1174).
The fruits of the milk thistle are typically used for preparing the extract. Such extracts from milk thistle and methods for preparing them have previously been described in the prior art, for example as disclosed in DE 1 923 982, DE 29 14 330 (Madaus).
Also known is a dried extract of milk thistle fruit (Extr. cardui mariae fruct. siccum) which is obtained from the plant drug using, among others, the extraction agent ethyl acetate, and standardized in accordance with the applicable Ph. Eur.
The stated requirements for a dry extract are a content of preferably 30-65% by weight silymarin (other content ranges are possible), the silymarin portion containing the following fractions:
40-65% by weight: Silibinin A and B (diastereomeric mixture, C25H22OH10Mw 482.4) and 10-20% by weight: Isosilibinin A and B (diastereomeric mixture, C25H22OH10Mw 482.4) and 20-45% by weight: Silidanin and silicristin (C25H22OH10Mw 482.4).
For preparation of an extract, the raw material (in this case, the plant drug) is usually degreased, extracted, filtered, concentrated, and purified. For said continuous extraction, using ethyl acetate/ethanol/acetone/methanol (optionally in aqueous form) or aqueous mixtures with the above-referenced solvents, filtration is usually performed, followed by concentration. Purification is then carried out using ethanol and hexane (further degreasing), thus obtaining the above-referenced content of silymarin. Such a composition allows a silymarin release rate of 30% to approximately 40% (measured in accordance with Ph. Eur. 5.7; 2.9.3 (01/2006:20903 as amended, for example using the basket or paddle method)).
However, there is a great need for increasing the release rate of silymarin, preferably silibinin, in the native extract.
Silibinin is lipophilic and thus, has a very poor solubility in aqueous liquids. It is known that flavonolignans, particularly silibinin, have little or no solubility in water (the solubility of pure silymarin is approximately 0.08 g/l at pH 6.9). Because of this solubility characteristic the release rate of flavonolignans, particularly silibinin, and de facto their bioavailability or absorbability in the body of humans or mammals, is inadequate.
In order to increase the release rate, attempts have been made to derivatize the flavonolignans, using polyalcohols, amino sugars, or esters, for example, or to complex them using inclusion compounds such as cyclodextrin (EP 0 422 497 B1 (Madaus)), or using complexing compounds, for example phosphatidylcholine. Such complexes, however, are not satisfactory in every respect and there is a need for medicaments that contain silibinin without requiring the presence of solubilizing complexes, such as phopholipid complexes or cyclodextrin inclusion complexes, and at the same time, providing a sufficient bioavailability of silibinin so that comparatively high plasma levels that are required for the treatment of viral hepatitis can be achieved upon oral administration.
It is also known from the prior art that the release rate may be increased by use of carrier substances such as 1-vinyl-2-pyrrolidone, mannitol, and others (EP 0 722 918 B1, U.S. Pat. No. 5,906,991 (Madaus)). In addition, wetting agents such as polysorbates (tensids) are necessary. EP 1 021 198 B1 (Madaus) discloses a silymarin coprecipitate with the use of PEG. Polar silibinin-esters are commercially available as an infusion solution, for example, under the name Legalon® SIL in the Federal Republic of Germany.
However, these referenced methods all have the disadvantage that dosing is made more difficult, and foreign substances may arise which may have imprecisely defined side effects.
L. Yu, Advanced Drug Delivery Reviews, 2001, 48, 27-42 relates to amorphous pharmaceutical solids, their preparation, characterization and stabilization.
WO 2009/080006 relates to a method for producing a milk thistle extract, particularly a flavonolignan preparation, having an increased release rate and improved resorbability, and to the use thereof, particularly for the therapy and prophylaxis of liver diseases.
U.S. Pat. No. 4,871,763 provides a process for the preparation of substantially pure silibinin from the fruits of Silybum marianum, as well as pharmaceutical compositions containing it for the treatment of diseases of the liver.
GB 2 167 414 discloses silibinin derivatives useful in pharmaceutical compositions for treating burn damage, liver damage or fungal poisoning.